This invention relates to an apparatus for detecting the breaking of a glass plate, and more particularly to such apparatus for detecting the vibration of a glass plate which occurs when the glass plate gets broken or damaged, so as to detect the breaking or damage of the glass plate.
It is known that when a glass plate having a piezoelectric element mounted thereon gets broken or damaged a mechanical vibration is produced. The electrical signal produced by the piezoelectric element due to mechanical vibration-to-electrical signal conversion has signal components not only in a low frequency range such as 20 to 30 kHz but also in a high frequency range higher than 100 kHz. Japanese laid-open patent publication (non-examined) NO. 48-66485/1973 based on British patent application No. 57598/71 discloses an apparatus which detects the breaking of a glass plate by detecting the converted electric signal with a frequency above 100 kHz only. According to the teaching of the above cited Japanese laid-open patent publication No. 48-66485/1973, the detection of signal components in a low frequency range lower than 100 kHz is avoided, so as to prevent erroneous detection of the breaking of the glass plate.
However, it has been found that signal components higher than 100 kHz appear at the output of the piezoelectric element not only when the glass plate gets broken but also when sand, a small stone, a small metal body or the like collides with the glass plate. This leads to an erroneous detection of the breaking of the glass plate